The invention concerns a novel supported catalyst particularly for use for selective gas phase hydrogenation of acetylenic hydrocarbon containing 2 or 3 carbon atoms to the corresponding ethylenic hydrocarbons. More particularly, it concerns a catalyst for selective hydrogenation of acetylenic compounds such as acetylene or propyne to ethylene or propylene respectively, in the gas phase.
It also concerns a selective hydrogenation process using this catalyst.
Ethylene is a monomer which is used in the preparation of a large number of polymers. It is generally obtained by pyrolysis or steam cracking of hydrocarbons. The ethylene which is produced contains small quantities of acetylene (generally less than 3%) which must be eliminated before use. The amount of acetylene which is generally tolerated in ethylene for use in the manufacture of polymers is generally less than 10 ppm, and usually less than 5 ppm.
One technique for eliminating acetylene in ethylene is to selectively hydrogenate it to ethylene in the presence of a palladium based catalyst supported on a refractory support such as alumina. The problem which is generally encountered with monometallic catalysts (constituted solely by palladium supported on alumina) is that when the operating conditions are such as to allow complete elimination of the acetylene, a portion of the ethylene is also converted to ethane. Further, such monometallic catalysts generally have relatively low stability due to the substantial formation of oligomers which gradually cover the surface of the catalyst under the reaction conditions. This hydrocarbon deposit can, of course, be eliminated by oxidation processes, but in an industrial process it is of advantage to have as long an operational time for the catalyst as possible between any two regeneration steps.
Addition of promoters to the palladium has long been proposed to improve the properties of the catalysts. Such additions can, for example, be silver (U.S. Pat. No. 2,802,889) or iron and silver (U.S. Pat. No. 3,243,387).
Such promoters can also be selected from alkali metals or alkaline-earth metals such as lithium (U.S. Pat. No. 3,325,556), potassium (European application EP-A-0 124 744) or calcium (U.S. Pat. No. 4,329,530).
Whether using monometallic catalysts (based on palladium alone) or promoted catalysts (comprising palladium and at least one other element), the skilled person is aware that when the palladium is concentrated at the surface of the catalyst particles (for example spherules), its catalytic performances are substantially superior to those of a catalyst with an identical formula where the palladium is homogeneously distributed in the catalyst particles. As an example, when using bimetallic palladium-silver formulae, it has been discovered that when the palladium is situated at the periphery of the catalyst spherules and the silver is homogeneously distributed, this improves the properties of the catalyst (U.S. Pat. No.4,404,124; EP-A-0 064 301 and French patent FR-A-2 597 113), in particular by a reduction in ethane formation and a reduction in oligomerisation product formation.
In addition, Japanese patent application JP-A-04 108540 describes selective liquid phase hydrogenation catalysts for 1-3-butadiene, in which the silver is precipitated and supported at the surface of the palladium. In such catalysts, the support consists of alumina with a relatively high specific surface area and the Ag/Pd ratio is 0.5:1 to 3.0:1 by weight, preferably 0.5:1 to 3.0:1 by weight.
In a prior patent application (FR-A-2 721 018, filed on 9.sup.th June 1994), a selective gas phase process is described for hydrogenating acetylenic hydrocarbons containing 2 or 3 carbon atoms (acetylene or propyne) to the corresponding ethylenic hydrocarbons (ethylene or propylene) using a catalyst in the form of spherules or extrudates containing palladium, at least one metal from group IB of the periodic classification, and alumina, in which a proportion of at least 80% of the palladium and a proportion of at least 80% of the group IB metal is present in a volume at the periphery of the catalyst delimited by a spherical or cylindrical surface with radius r.sub.1 corresponding to the average radius of the catalyst spherules or extrudates and a spherical or cylindrical surface with radius r.sub.2 which is at least equal to 0.8 r.sub.1. The silver/palladium ratio is in the range 0.05 to 0.4 by weight, preferably in the range 0.05 to 0.25 by weight.
More particularly, the palladium content is in the range 0.01% to 0.05% by weight of the catalyst. The group IB element, normally silver, is present in an amount in the range 0.001% to 0.02% by weight of catalyst.
For the industrial application envisaged, the catalyst performances must vary as little as possible over time. In fact, a catalyst for which acetylene is firstly totally converted but in which the acetylene would appear very rapidly at the reactor outlet at high concentrations, could not be used as it would cause considerable control problems in the industrial unit. This means that one of the selection criteria for the hydrogenation catalyst is its deactivation, which corresponds to the speed at which acetylene appears at the reactor outlet. Such deactivation must be as long as possible.